As COVID-19 infection rates start to slow in some countries around the world, several governments are now figuring out how to protect citizens in the long-term. Many countries, such as China, Italy and the US, are looking at how and when to lift lockdowns safely to allow people to return to work.

But what exactly is herd immunity and how does it work? Professor Willem van Schaik, professor of microbiology and infection at the University of Birmingham, has described herd immunity and how it could affect the coronavirus pandemic.

What is herd immunity?

In theory, herd immunity means not everyone in a community needs to be immune to prevent the spread of a disease. ‘Herd immunity describes the phenomenon that at-risk individuals are protected from infection because they are surrounded by immune individuals. The spread of the virus is thus minimised,’ Prof van Schaik says.

How does herd immunity work?

Herd immunity describes the resistance to the spread of a contagious disease within a population due to a sufficiently high proportion of individuals being immune either through vaccination or by developing immunity after infection.

‘Currently, we talk mostly about herd immunity in the context of vaccines. If a sufficiently high number of individuals in a population are vaccinated, they will provide herd immunity to the small number of people that are not vaccinated,’ says Prof van Schaik.

Herd immunity in the UK

In March, the government’s chief scientific advisor Sir Patrick Vallance said that about 40 million people in the UK may need to catch COVID-19 to build up herd immunity in order to manage the spread of the infection and prevent the disease from coming back in the future.

Health secretary Matt Hancock denied the government was taking this approach.

What percentage is needed for herd immunity?

Prof van Schaik has expressed his concerns about the success of using herd immunity to curb the rate of infection in the UK.

‘Unfortunately, a very rough estimate suggests that we will only reach herd immunity to COVID-19 when approximately 60 per cent of the population is immune (and remember that immunity is currently only reached by getting the infection as we have no vaccine),’ Prof van Schaik says.

What are the risks of herd immunity?

The professor expressed serious concerns about the risk implications involved. ‘The major downside is that this will mean that in the UK alone at least 36 million people will need to be infected and recover. It is almost impossible to predict what that will mean in terms of human costs but we are conservatively looking at tens of thousands of deaths, and possibly at hundreds of thousands of deaths.’

‘The only way to make this work would be to spread out these millions of cases over a relatively long period of time so that the NHS does not get overwhelmed.’

Herd immunity examples

Herd immunity currently exists for the flu, thanks to large proportions of the population getting the flu vaccine that can protect non-immunised individuals. However, given there are different strains of the flu, it is not always 100 per cent effective.

COVID-19 is unique in that it is a novel virus, meaning that everyone is currently at risk of infection. ‘Herd immunity can only be reached by widespread vaccination (but there is currently no vaccine, and it may take a long time before an effective vaccine becomes available) or by individuals falling ill or recovering thereby developing natural immunity against the virus,’ Prof van Schaik says.

How effective is herd immunity?

While it is true that herd immunity makes it harder for a disease to spread due to people becoming immune after already catching it or getting vaccinated, there are ‘considerable risks and downsides’ of letting a potential lethal infectious disease burn through a population in order to reach herd immunity.

‘This is why we still need to all work together to try to slow down and minimise the spread of COVID-19 by practicing hand hygiene and social distancing. This will hopefully reduce the pressure on the NHS, allowing for more people to be treated and thus reducing the number of people dying of this infection,’ advises Prof van Schaik.

Potential problems

The problem with trying to work out when herd immunity will eventually arise and be successful is that we do not know exactly how our long-term immunity to COVID-19 will work. It is not clear whether those who have had mild symptoms develop as strong an antibody immune response as those with stronger symptoms. Also for all statistical modelling we need to know how long our immunity will last after having this particular corona virus. We also believe at present that even in the hardest hit areas it is still a minority of the world’s population that have confirmed infection. With further testing we will know more and hope that herd immunity will build, but we still have a long way to go to reach the 60 per cent. Maintaining the ‘flattening of the curve’ by social distancing and isolation so that intensive care beds and the health services can cope with the impact, is currently still of paramount importance in the interim.